Observing the manner in which a beam of light interacts with an aerosol, with the degree of extinction of the incident beam and the angular distribution of the scattered light, can be a powerful technique for inferring properties of the aerosol particles, such as their shapes, sizes, and numbers. In the past 40 years, concurrent with the increasing power and availability of digital computers, it has become practical to carry out the extensive calculations which relate particle properties to light scattering properties, thus spuring the development of instruments for measuring light scattering.
A feature common to all such instruments is that the aerosol and light beam must be brought together. Numerous aerosol handling systems have been devised, varying in their operation according to the primary measurement goals of the instruments in which they are included.
Aerosol sampling systems for light scattering instruments may be conveniently differentiated according to the quantity of aerosol that is illuminated and examined. Devices intended to measure the extinction undergone by a light beam traversing an aerosol generally benefit from path lengths of several meters or more, so relatively large volume of aerosol must be illuminated, even hundreds of cubic centimeters. In these cases aerosol isn't sampled and delivered to the light beam so much as the instrument is delivered and set up at (or in) the location of the cloud. Examples include the Barnes model 14-70B transmissometer, and the Malvern Particle Sizer which examines the angular distribution of very nearly forward scattered light.
More compact instruments, designed to measure scattering only, draw a sample of aerosol and direct it across a collimated or focused light beam. The beam/aerosol intersection occurs inside an otherwise clean chamber and typically occupies a volume on the order of a cubic centimeter. Instruments delivering aerosol in this fashion include the Climet CI-261 Aerosol Nephelometer and the Sinclair-Phoenix Aerosol Photometer, both of which measure angularly integrated forward scattered light to indicate aerosol concentration, and NASA's polar nephelometer (Applied Optics 9 1113, 1970) in which a detector is scanned in a semi-circle to measure the angular dependence of light scattered from an illuminated column of sample aerosol.
The smallest quantity of aerosol is illuminated in optical particle counters, in which the light beam, aerosol stream, and detection optics are so tightly focused that, ideally, only one particle at a time produces a detectable light scattering signal. Often these instruments use a concentric pair of tubes as the inlet nozzle, an arrangement known as a sheath nozzle. Sample aerosol enters the scattering chamber through a narrow inner tube, while an outer tube, sometimes tapered, delivers a clean surrounding sheath flow that confines the sample particles to a narrow central core and directs them through a small but intense region of illumination.
The Royco 220 Particle Counter uses a laminar sheath flow to guide particles through a focused region of white light; light scattered from the particles into a lens at a right angle to the incident beam is detected. The Climet CI-208 Airborne Particle Analyzer also uses a sheath air nozzle, but operates at very high (turbulent jet) gas velocity rather than laminar flow. Aerosol particle/light interaction takes place at one focus of an elliptical mirror; nearly all the scattered energy can be collected by a detector at the other focus.
Full optical characterization of an aerosol cloud requires measurements of its extinction and its angular scattering properties, including polarization characteristics of the angularly scattered light. Current instruments do not combine the seemingly incompatible requirements of very long path lengths demanded for extinction measurements and the localization of illuminated sample needed for angular measurements and the efficient use of light gathering optics.
Transmissometers do not have angular scanning capability; such an instrument would be immense. The extinction of light in nephelometers, built for angular scanning, is too slight to be accurately measured. It is typically much smaller than the natural fluctuations of the light source itself. Single particle instruments, in addition to the extinction problem, are designed to analyze single particle characteristics and must process very low signal levels. The optical properties of a cloud as a whole could be obtained by summing thousands of individual particle measurements, but that is clearly an imprecise and inefficient procedure whenever only ensemble properties are desired.
Accordingly, it is an object of this invention to provide an improved nozzle for use with instruments designed to measure light scattering properties of aerosols.
More particularly, it is an object of this invention to provide a new nozzle which delivers a pulsating concentration of aerosol particles to a very small illuminated volume, thus enabling simultaneous measurements of extinction and scattering.
Other objects will appear hereinafter.